This research project is part of a continuous effort to gain insight into the maturational process of renal function by use of micropuncture methodology. Its specific aims are to investigate some of the mechanisms controlling glomerular filtration, to explore the relationship between filtration and reabsorption and the permeability characteristics of the tubule during development. The effect of acute stress on the factors involved in the process of glomerular ultrafiltration will be determined in Munich-Wistar rats by using a servo-null pressure device. A similar methodology will be employed for chronic studies of adaptation to loss of renal parenchyma. The characteristics of the glomerulo-tubular balance in immature guinea pigs will be investigated under conditions of aortic constriction. The relationship between the ability to filter and the capacity to reabsorb will be measured for substances other than salt and water, such as bicarbonate and glucose. Finally, the permeability characteristics of the tubule to solutes and water will be assessed in an attempt to explain the ability of the proximal tubular wall to withstand a higher hydrostatic pressure gradient in newborn than in adult animals.